A modem translates digital data originating in a digital device, such as a computer, to an analog signal so that it can be transmitted on an analog telephone line to another digital device. The modem also translates the incoming analog signal originating in the other digital device to digital data for the digital device. Thus, the analog telephone line can be used to transmit data between digital devices.
In order for a modem to transmit data to another modem, the calling (originating) modem dials the telephone phone number of the answering (terminating) modem. The General Switched Telephone Network (“GSTN”) (the Public Switched Telephone Network, (“PSTN”), is an instance of a GSTN) rings the answering modem indicating an incoming call. The answering modem goes off-hook to accept the call. A GSTN switched connection now exists between the calling modem and the answering modem. The modems go through a series of training steps, to adjust to each other's characteristics, and to the characteristics of the switched connection. These training steps can include Answer Back Tone (“ABT”) and other modem detect signals, V.8, V.8bis, auto-mode, and the negotiation of error correction and data compression. These steps are defined by the International Telecommunications Union (“ITU”) V series of modem recommendations and other modem specifications including Flex and MNP5 compression. ITU V series modem recommendations include, for example, V.8, V8bis, V.42, V.42bis, V.21, V.22bis, V.32, V.32bis, V.34, V.90 and V.92.
Real-time audio, such as a telephone conversation originating on an analog telephone line, may be transmitted over a packet network such as, the Internet using Voice over Internet Protocol (“VoIP”). VoIP may be used instead of a GSTN switched connection in order to avoid incurring charges for a long distance telephone call. One standard VoIP protocol for encapsulating real-time audio data is the Real-Time Transport Protocol (“RTP”) (Request for Comments (“RFC”) 1889, January 1996) available on the Internet Engineering Task Force (“IETF”) web site.
To transmit voice data over the Internet, a packet transmitter in the source Internet node encodes the analog voice signal, stores the encoded data in the payload of one or more data packets, and transmits the data packet over the Internet. Each data packet includes a destination address in a header included in the data packet.
Modem and Voice transport services are inherently part of a traditional GSTN network. Thus, a VoIP gateway must support both modem and voice. One known method for supporting modem service over VoIP transport is called modem pass-through. Modem pass-through emulates a GSTN network when transporting a G.711 Pulse Code Modulation (“PCM”) stream. To implement modem pass-through, VoIP sets its encoder/decoder (CODEC) to PCM, disables the echo canceler and fine tunes the voice play out for handling de-jitter of arriving RTP data packets.
Voice activity detection (“VAD”) is a method used by VoIP to detect the start of a period in which audio received from the GSTN network does not contain spoken words. The period in which the audio does not contain spoken words is sometimes called “a period of silence”. Upon detecting a period of silence, the transmitter gateway stops transmitting data packets onto the VoIP network. During the period of silence, the remote gateway generates comfort noise and transmits the comfort noise to the GSTN network. However, when transmitting data between modems, there is no “period of silence” while the connection is idle, because of the requirement to keep the connection between the modems alive. Thus, using VoIP to transfer data between modems consumes Internet bandwidth by transferring unnecessary idle data packets over the Internet while the connection is idle.